Wireless networks have experienced increased development in the past decade. One of the most rapidly developing areas is mobile ad-hoc networks (MANETs). Physically, a MANET includes a number of geographically-distributed, potentially mobile nodes sharing one or more common radio channels. Compared with other types of networks, such as cellular networks or satellite networks, the most distinctive feature of MANETS is the lack of any fixed infrastructure. The network is formed of mobile and stationary nodes, and is created on the fly as the nodes communicate with each other. The network does not depend on a particular node and dynamically adjusts as some nodes join or others leave the network.
In a hostile environment where a fixed communication infrastructure is unreliable or unavailable, such as in a battle field or in a natural disaster area struck by earthquake or hurricane, a MANET can be quickly deployed to provide much needed communications. While the military is still a major driving force behind the development of these networks, ad-hoc networks are quickly finding new applications in civilian or commercial areas. MANETs will allow people and applications to exchange data in the field or in a class room without using any network structure except that which they create by simply turning on their computers or PDAs.
As wireless communication increasingly permeates everyday life, new applications for MANETs will continue to emerge and become an important factor in wireless communications. Yet, MANETs pose serious challenges to designers. Due to the lack of a fixed infrastructure, nodes must self-organize and reconfigure as they move, join or leave the network. All nodes are essentially the same, and there is no natural hierarchy or central controller in the network. All functions have to be distributed among the nodes. Nodes are often powered by batteries and have limited communication and computation capabilities. Also, the bandwidth of the system is usually limited. The distance between two nodes often exceeds the radio transmission range, and a transmission may have to be relayed by other nodes before reaching its destination. Consequently, a MANET network typically has a multi-hop topology, and this topology changes as the nodes move around.
The MANET working group of the Internet Engineering Task Force (IETF) has been actively evaluating and standardizing routing protocols, including multicasting protocols. Because the network topology changes arbitrarily as the nodes move, information is subject to becoming obsolete, and different nodes often have different views of the network, both in time (information may be outdated at some nodes but current at others) and in space (a node may only know the network topology in its neighborhood and not far away from itself).
A routing protocol needs to adapt to frequent topology changes, possibly with less than accurate information. Because of these unique requirements, routing in these networks is very different than in others. Gathering fresh information about the entire network is often costly and impractical. Some routing protocols are reactive (i.e., on-demand) protocols. That is, they collect routing information only when necessary and only to destinations to which they need routes, and do not maintain unused routes. In this way the routing overhead may be reduced compared to pro-active protocols, which maintain optimal routes to all destinations at all time. Ad Hoc on Demand Distance Vector (AODV), Dynamic Source Routing (DSR) and Temporally Ordered Routing Algorithm (TORA) are representatives of reactive routing protocols presented at the MANET working group.
An example of a proactive routing protocol is Optimal Link State Routing (OLSR). Examples of other various routing protocols include Destination Sequenced Distance-Vector (DSDV) routing which is disclosed in U.S. Pat. No. 5,412,654 to Perkins, and the Zone Routing Protocol (ZRP) which is disclosed in U.S. Pat. No. 6,304,556 to Haas. ZRP is a hybrid protocol using both proactive and reactive approaches.
These conventional routing protocols use a best effort approach in selecting a route from the source node to the destination node. Typically, minimizing the number of hops is the main criteria in such approaches.
Quality-of-service (QoS) routing in MANETs is gaining interest. To provide quality-of-service, a protocol needs not only to find a route but also to identify and/or secure the resources along the route. Because of the potentially limited, shared bandwidth of the network, and the lack of a central controller which can account for and control these limited resources, nodes must negotiate with each other to manage the resources required for QoS routes. This is further complicated by frequent topology changes. Due to these constraints, QoS routing is more demanding than best-effort or minimum-hop routing.
Some examples of QoS routing approaches are set forth by Chenxi Zhu in the publication entitled “Medium Access Control and Quality-of-Service Routing for Mobile Ad Hoc Networks,” 2001, and by M. Mirhakkak et al. in the publication entitled “Dynamic Quality-of-Service for Mobile Ad Hoc Networks,” MITRE Corp., 2000. Zhu discusses establishing bandwidth guaranteed QoS routes in small networks whose topologies change at a low to medium rate. Mirhakkak et al. are concerned with resource reservation requests which specify a range of QoS values while the network makes a commitment to provide service within this range.
Since MANETs are still in the initial stages of development, most attempts to implement QoS functionality in MANETs thus far have focused primarily on using QoS parameters to establish routes, as is the case with some of the above-noted prior art approaches. Yet, as MANETs continue to increase in size and complexity, further QoS functionality may be needed along with ways to level loads in the network to reduce delays and optimize energy usage.